A rock originally had 4800 atoms of 40K and no 40Ar. It now has 400 atoms of 40K.
Assuming that the only chemical process is radioactive decay of potassium to
argon, what is the age of the rock?
a) 1.576 billion years
b) 1.375 billion years
c) 2.643 billion years
d) 4.660 billion years
e) 5.303 billion years
How many atoms of 40Ar are there in the rock now?